題目

Rotate an array of n elements to the right by k steps.

For example, with n = 7 and k = 3, the array [1,2,3,4,5,6,7] is rotated to [5,6,7,1,2,3,4].

Note:
Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.

題目大意

旋轉陣列，給定陣列長度n，旋轉位移k，將陣列元素迴圈右移k位。

思路

三步翻轉法，《程式設計珠璣》一書上有講，步驟如下：

1. 將陣列從k位置分為前後兩半；
2. 翻轉前半部分；
3. 翻轉後半部分；
4. 翻轉整個陣列；

圖示如下：

解答

Java程式碼如下，注意遍歷時候的邊界不要出錯：

    public void rotate(int[] nums, int k) {
       if(k==0 || nums==null ) return;
        k = k%nums.length;
        int tmp;
        for (int i = 0; i < (nums.length-k)/2; i++) {
            tmp = nums[i];
            nums[i] = nums[nums.length-k-1
 
-i];
            nums[nums.length-k-1-i] = tmp;
        }
        /*for (int i : nums) {
            System.out.print(i+"-");
        }
        System.out.println();*/

        for (int i = 0; i < k/2; i++) {
            tmp = nums[i+nums.length-k];
            nums[i+nums.length-k] = nums[nums.length-1
 
-i];
            nums[nums.length-1-i] = tmp;
        }

        /*for (int i : nums) {
            System.out.print(i+" ");
        }
        System.out.println();*/

        for (int i = 0; i < nums.length/2; i++) {
            tmp = nums[i];
            nums[i] = nums[nums.length-1-i];
            nums[nums.length-1-i] = tmp;
        }
    }